Dear Stefano, That is very helpful thank you, I will try out your strategy now.
I did have one more question if I may ask it. Suppose I have single unit cell FCC and I converge the energy cutoff and k-points. If I then set up a 2x2x2 supercell of that unit cell, would I have to repeat the energy cutoff and k-point convergence for the larger supercell? All the best, Ben Palmer, Student @ University of Birmingham, UK > Dear Ali Kachmar, > > convergence w.r.t. ecutwfc (and ecutrho) and convergence w.r.t. > k-points sampling are rather independent issues and can be tested to a > large extent separately > > - convergence w.r.t. ecutwfc and ecutrho is a property depending on > the highest Fourier components that are needed to describe the > wavefunctions and the density of your system. his depends on the > pseudopotentials that are present in the calculation and do not depend > strongly, for a given set of pseudopotentials, on the particular > configuration because it depends mostly on the behaviour of the wfc in > the core region which is quite insensitive (in terms of shape) on the > environment. > So each pseudopotential has a required cutoff. An upperbound to this > value can be determined from any system that contains that pseudo. > The cutoff needed for a system containing several species is the > highest among those needed for each element. > Moreover, in US pseudo or PAW the charge density has contributions > from localized terms that may (an usually do in USPP) require quite > higher cutoff than the one needed for psi**2 (4*ecutwfc) ... hence the > possibility to vary and test independently for ecutrho ... > > My recommended strategy to fix ecutwfc and ecutrho is to perform total > energy (and possibly, force and stress) covergence test increasing > ecutwfc keeping ecutrho at its default vaule (=4*ecutwfc) until > satisfactory stability is reached (typically ~1 mry/atom in the > energy, 1.d-4 ry/au in the forces, a fraction of a KBar in the stress) > ... this fixes the converged value of ecutrho to 4 times the > resulting ecutwfc. > Now keeping this value for ecutrho one can try to reduce ecutwfc and > see how much this can be done without deteriorating the convergence. > > -convergence with respect to k-points is a property of the band > structure. > I would study it after the ecutwfc/ecutrho issue is settled but some > fairly accurate parameters can be obtained even with reasonable but > not optimal cutoff parameters. > > There is a big difference between convergence in a band insulator or > in a metal. > > In an insulator bands are completely occupied or empty across the BZ > and charge density can be written in terms of wannier functions that > are exponentially localized in real space. > Hence the convergence w.r.t the density of point in the different > directions in the BZ should be exponentially fast and anyway quite > quick... > > In a metal the need to sample only a portion of the BZ would require > an extremely dense set of k points in order to locate accurately the > Fermi surface. This induces to introduce a smearing width that smooth > the integral to be performed... the larger the smearing width, the > smoother the function, and the faster the convergence results... > however the larger the smearing width the farther the result is going > to be from the accurate, zero smearing width, result that one would > desire. > Therefore different shapes fro the smearing functions have been > proposed to alleviate this problem and > Marzari-Vanderbilt and Methfessel-Paxton smearing functions give a > quite mild dependence of the (k-point converged) total energy as a > function of the smearing width thus being good choices for metals. > > My recommended strategy for fix the k-point sampling in metals is > 1) chose the smearing function type (mv or mp, recomended) > 2) for decreasing values of the smearing width (let's say from an high > value of 0.1 ry = 1.36 eV to a low value of 0.01 - 0.005 ry = > 0.136-0.068 eV if feasable) CONVERGE the total energy w.r.t to > smearing well within the global desired tolerance (of 1 mry/atom, for > instance) > 3) by examining the behaviour of the CONVERGED Energy vs smearing > width curve E(sigma) identify the smearing width for which E(sigma) is > within tolerance w.r.t. E(sigma==0) keeping in mind that for > methfessel-paxton E(sigma) ~ E(0) + A*sigma**4 + o(sigma**6) while for > marzari-vanderbilt the dependence is more likely E(sigma) ~ E(0) > +A*sigma**3 + o(sigma**4). > 4) select that value of the smearing width and the smallest set of > k-points for which this is converged. > > HTH > > stefano > > > > On 02/24/2013 06:54 PM, Ali KACHMAR wrote: >> Hi, >> >> as far as I know, there is no any techinques for choosing ecut and k-points. >> Please have a look at the pwscf archive and make up a conclusion. >> >> Best, >> Ali >> >>> Date: Sat, 23 Feb 2013 19:55:51 +0000 >>> From:benpalmer1983 at gmail.com >>> To:pw_forum at pwscf.org >>> Subject: [Pw_forum] Technique for converging Ecut and K-points? >>> >>> Hi everyone, >>> >>> I just wanted to ask if users have any techniques for choosing ecut and >>> k-points? I've read that one way would be to start with a high number >>> of k-points and high energy cutoff, and use that energy as an almost >>> true value. Then adjust k-points and energy cutoff from a lower >>> number/cutoff until it converges to the true value. Would you try to >>> converge energy cutoff first, or k-points? Does it matter which you >>> converge first? >>> >>> Thanks >>> >>> Ben Palmer >>> Student @ University of Birmingham >>> _______________________________________________ >>> Pw_forum mailing list >>> Pw_forum at pwscf.org >>> http://pwscf.org/mailman/listinfo/pw_forum >> >> >> >> _______________________________________________ >> Pw_forum mailing list >> Pw_forum at pwscf.org >> http://pwscf.org/mailman/listinfo/pw_forum > > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://pwscf.org/mailman/listinfo/pw_forum -------------- next part -------------- An HTML attachment was scrubbed... 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